Since standard optical techniques do not allow the inspection of interior sections, for example of an organ in the human body or stresses or cracks in the interior of a metal or other solid part, alternate techniques including x-ray and radiographic methods, eddy current testing in metals, the use of dye penetrants, and, more recently, utilization of acoustic imaging methods have been employed.
Radiographic techniques require bulky apparatus, particularly for thick metal parts, and high energy beams must be employed so as to require clearing of the area where the inspection is being made.
The eddy current techniques also provide very useful information, but only on near-surface defects in metals; and, in turn, fluorescent dye penetrants provide a simple and effective way of detecting surface defects, but do not necessarily evaluate accurately the depth of the crack or any of its characteristics.
As a consequence, acoustic techniques have been increasingly utilized, as they can readily penetrate the otherwise opaque bodies to measure the subsurface elastic properties of the material. For example, an acoustic microscope has been developed wherein the acoustic energy is focused by a spherical lens and the sample is scanned by means of a loudspeaker movement in one direction and by a hydraulic piston in the perpendicular direction. Such mechanical scanning of a region point by point is relatively slow and provides a major disadvantage of the arrangement.
Some attempts have been made, as a consequence, to provide electronically variable focusing techniques rather than the physical lens, to provide much more rapid scanning of an object. One such system provides for an array of electroacoustic transducers where the echo or return signals from the object under inspection are passed through individual lumped electromagnetic delay lines which can be appropriately designed to vary the time delay for each transducer, and thus provide an electronic focusing scheme. However, as a practical matter, the lumped time delay lines require a great deal of space and cumbersome switching circuits. Switching of this time delay system is controlled by a minicomputer, or microprocessor.
Other computer reconstruction techniques have been utilized in connection with acoustic imaging by other investigators. However, a number of practical problems exist, such as poor resolution of the image, and, most importantly, inability to provide for signal acquisition and reconstruction of the image in real time.